The carbonic acid/bicarbonate conjugate acid-base pair are the predominant buffer in the plasma and glomerular ultrafiltrate. Most of the filtered bicarbonate is reabsorbed by the kidney, and this process is critically important in regulating the acid-base balance of the whole organism. H ion secretion into the tubule lumen is thought to mediate most bicarbonate reabsorption. The generation and maintenance of considerable bicarbonate gradients occur concomitantly with the secretion of H ion into the tubule lumen. The transepithelial anion gradients developed by H ion secretion may also play an important role in the reabsorption of salt and water by the kidney. The objectives of this proposal include the study of a) CO2 movement and bicarbonate reabsorption across renal tubule epithelium, b) the mechanisms of H ion secretion into the tubule lumen and c) the intrinsic CO2 and bicarbonate transport capacities of specific segments of the nephron. These studies will be done with isolated rabbit renal tubules perfused in a physiologic bathing medium in vitro. This arrangement permits the collection of perfusates from the tubules where both the initial perfusate and bath composition are controlled. The total CO2 contents of perfusates will be measured microcalorimetrically. Glass microelectrodes will be used to measure pH and pCO2 of collected perfusates. Ionic permeabilities will be determined with standard isotopic and electrophysiologic techniques. The studies shall explore the transepithelial movement of CO2 and bicarbonate. This information is needed to understand the regulation of acid-base balance and the mechanisms of H ion secretion at the level of the individual renal tubule.